1. Field of the Invention
The present invention relates to a sheet-shaped printing blanket which is used for winding around a blanket cylinder of an offset printing press.
2. Description of the Prior Art
The printing blanket generally has a structure wherein a surface printing layer made of an elastomer such as rubber is laminated on at least one base fabric.
To cope with an increase in speed of printing presses and an improvement in quality of printed images, there has recently been popularized a so-called air-type printing blanket having a porous compressible layer made of an elastomer such as rubber, for instance, which is interposed between the above base fabric and surface printing layer.
The above air-type printing blanket is lower in compressive stress in a nip deformed portion produced by being pressed against a plate cylinder, as compared with a conventional printing blanket having no compressible layer (normally referred to as a solid-type printing blanket) and is also lower in a fluctuation in compressive stress with respect to the change in amount of distortion at the above nip deformed portion. Therefore, the air-type blanket is generally high in impact absorbability.
Accordingly, the air-type blanket is superior in preventing impact produced by the feeding gears of the printing press or impact produced at the time when the joint of the blanket wound around the blanket cylinder passes through the pressed portion against the plate cylinder, for example, from affecting printing precision.
The solid-type printing blanket causes a so-called bulge by stress concentrations on the surface printing layer in the nip deformed portion, which might result in inferior printing such as out of register due to expansion in the circumferential direction, inferior paper feeding, double, or deformation of a dot pattern (particularly, dot gain).
On the other hand, the air-type printing blanket also has the effect of preventing the above-mentioned inferior printing because the compressible layer has the function of lowering stress concentrations on the surface printing layer, thereby inhibiting expansion of the surface printing layer in the circumferential direction.
The examples include a compressible layer having a closed cell structure in which voids are independent of each other, which is formed by (a) foaming matrix rubber constituting the compressible layer by an expanding agent which is decomposed by heating to emit gas, or (b) blending a hollow microsphere with matrix rubber, for example, and a compressible layer having an open cell structure in which voids connect with each other, which is formed by (c) a so-called leaching method for dispersing particles such as common salt particles, extractable by a solvent (water in the case of the common salt particles) which does not affect rubber, in matrix rubber, vulcanizing the matrix rubber, and then extracting the particles.
In order to form the compressible layer, however, a lot of complicated steps are required even in the printing blanket having either one of the structures as described above and, furthermore, the size of the cell structure is liable to vary. Therefore, the air-type printing blanket is lower in productivity than the solid-type printing blanket.